1. Field of the Invention
The present invention generally relates to the field of high intensity illumination devices, and more specifically to a high-power xenon-arc searchlight which has a continuous vertical beam direction range from straight down through straight up.
2. Description of the Related Art
Xenon-arc lamps provide an efficient source of high intensity illumination for a diverse range of applications, including light sources for cinematography and mobile searchlights carried by helicopters. An exemplary xenon-arc lamp application is disclosed in U.S. Pat. No. 3,720,822, entitled "XENON PHOTOGRAPHY LIGHT", issued Mar. 13, 1973 to J. Rochester et al.
The main elements of a photography light 10 as disclosed by Rochester are illustrated in FIG. 1. A xenon-arc lamp 12 includes a quartz tube 14 which is filled with xenon gas and has a longitudinal axis 16. An anode 18 and a cathode 20 are disposed inside the tube 14 and spaced from each other along the axis 16. An anode contact 22 and a cathode contact 24 enable connection of the anode 18 and cathode 20 respectively to an external direct current (DC) power source (not shown).
Upon application of DC power to the lamp 12, the xenon gas in the tube 14 is ionized and a high intensity luminous arc 25 is formed between the anode 18 and cathode 20 having maximum intensity at a point 26. A concave reflector 28 having a reflecting surface 28a with a parabolic, elliptical, aconic, spherical, or other suitable cross-section is mounted relative to the lamp 12 such that the reflecting surface 28a faces the lamp 12 and the central axis of the cross-section of the reflector 28 coincides with the axis 16 of the lamp 12. A reflecting layer 28b of aluminum or rhodium is formed on the surface 28a. Alternatively, although not shown, the reflector 28 may be transparent, and a reflecting layer formed on the rear surface of the reflector 28. The anode 18 is disposed between the reflector 28 and cathode 20.
Assuming that the reflecting surface 28a has a parabolic cross-section with a focus 30, the lamp 10 produces a narrow or tightly focussed beam when the reflector 28 is in the position illustrated such that the focus 30 of the reflecting surface 28a coincides with the point 26 of maximum intensity of the arc 25. Light from the arc 25 is collected by the reflector 28 as indicated by arrows 32, and reflected out of the lamp 10 along (generally parallel to) the axis 16 as a beam indicated by arrows 34.
FIG. 2 illustrates how the light 10 can be manipulated to produce a wider, less focussed beam, and plots the luminous intensity of the arc 25 as a function of displacement from the point 26 for a typical xenon-arc lamp 12. The curves indicate the luminous intensity in candela per square centimeter. It will be seen that in the illustrated example, the luminous intensity is 2,260 at the point 26, and decreases as a function of displacement from the point 26 toward the anode 18 to a value of 150 adjacent to the anode 18.
In the solid line position of the reflector 28 as shown in FIG. 2, the focus 30 of the reflecting surface 28a coincides with the point 26 of maximum intensity of the arc 25, and the light 10 radiates a beam with maximum focus and minimum width. The actual beam width varies with the size of the lamp 12 and the type of reflector 28. For an exemplary lamp 12 having a power rating of 500 watts and a parabolic reflector 28 having a focal length of 1.9 cm, the minimum width beam will have a divergence on the order of 1.degree..
The focus can be progressively reduced and the beam made progressively wider by moving the reflector 28 upwardly toward a broken line position as indicated at 28'. In this case, the focus, here designated as 30', is closer to the anode 18 than in the position 26, such that the reflector 28' collects light from a larger portion of the arc 25 and produces a wider beam with divergence on the order of 12.degree..
The focus and beam width are continuously variable between approximately 1.degree. and 12.degree. in the manner described. It is also possible to position the reflector 28 and lamp such that the cathode 18 is disposed between the reflecting surface 28a and the anode 20. In this case, the beam is defocussed by moving the reflector 28 toward the lamp 12, opposite to the operation described with reference to FIG. 2. However, this arrangement is less desirable since possible range of focus is smaller, on the order of 1.degree. to 6.degree..
The prior art configuration illustrated in FIGS. 1 and 2 is satisfactory for lights with low-power (less than approximately 300 watt) xenon-arc lamps, and applications such as helicopter-mounted searchlights which are only required to direct their beams from slightly above horizontal to vertically downward. Low-power xenon-arc lamps will operate at any orientation. However, a higher-power xenon-arc lamp becomes inoperative if oriented such that the anode 18 is disposed below the cathode 20 by more than a small distance. More specifically, the arc 25 will become unstable or extinguish if the anode 18 is disposed below the cathode 20, and the longitudinal axis 16 is inclined by more than a predetermined angle .theta., typically on the order of 15.degree., from the horizontal.
The operating range of a conventional high-power xenon-arc 10 is illustrated in FIGS. 3a to 3c. FIG. 3a illustrates one extreme operative orientation in which the anode 18 is leftward of and below the cathode 20, and the axis 16 is inclined by the angle .theta. from the horizontal which is indicated at 36. FIG. 3b illustrates the ideal operating condition of the light 10, rotated 105.degree. clockwise from the position of FIG. 3a, in which the anode 18 is disposed directly above the cathode 20. FIG. 3c illustrates the opposite extreme operating condition of the lamp 10, rotated 105.degree. clockwise from the position of FIG. 3b, in which the anode 18 is rightward of and below the cathode 20 and the axis 16 is inclined by .theta. from the horizontal 36.
The prior art light 10 is thereby operative with a vertical or elevation range of 210.degree., extending from 15.degree. above the horizontal 36 in one direction, through vertically downward to 15.degree. above the horizontal 36 in the opposite direction. However, numerous applications require a searchlight having an unlimited range of vertical beam direction, extending from straight up through horizontal to straight down.
The requirement that the anode of a xenon-arc lamp not be oriented below the cathode can be satisfied while providing a full vertical range of beam direction by mounting the lamp horizontally and rotating the reflector in a vertical plane which is perpendicular to the axis of the lamp. Although this arrangement is acceptable in applications in which the beam width is maintained in the most narrowly focussed state, attempts to provide a wider beam width result in an extremely asymmetric beam.
It is also possible to maintain the lamp vertical or horizontal, and rotate the reflector about the lamp using a gimbal arrangement. However, this is difficult and expensive to embody in actual practice, since the displacement of the lamp and reflector between the minimum and maximum beam width positions is very small, on the order of 4-5 millimeters. The mechanical tolerances of a gimbal mechanism required to maintain a fixed beam width over the entire vertical beam range are very close and expensive to achieve and maintain under conditions such as encountered by helicopter and other ground, airborne and marine vehicle-mounted searchlights which are subject to heavy vibration. In addition, the gimbal arrangement is usable for only relatively small lamps, since larger lamps mechanically interfere with the movement of the reflector and gimbal mechanism.
The reflecting layer 28b of the reflector 28 has conventionally been formed of aluminum, silver, rhodium or multi-layer dielectric materials. However, these materials have various disadvantages. Aluminum has poor resistance to atmospheric corrosion. Silver tarnishes quickly upon exposure to air, and for this reason can only be used on the rear surface of the reflector 28. Rhodium is extremely expensive, and can only be used in thin layers which are sensitive to atmospheric conditions and easily damaged by cleaning. Multi-layer dielectrics only reflect light in a narrow wavelength band, are expensive to produce, and are also easily damaged by cleaning.
Gold has been used as reflecting material in infrared optical systems. However, it has not been employed in visible optical systems since it has relatively low reflectivity in the shorter visible wavelengths, notably the blue region. Although more expensive than aluminum and silver, gold is much less expensive than rhodium, and is highly resistant to tarnish and corrosion.